sinking in my dream

I had a dream a couple of nights ago. I was a passanger in a car that was driving on a highway that wrapped around the side of a mountain with the mountain wall on the left and a large body of water down below on the right. I looked in the water and saw a semi truck (just the truck, no cargo attached) floating there. I began to sink. Then a crain from the shore nearby reached into the water and grabbed the truck just as it was submerged and pulled it to shore. A great relief swept over me. Then suddenly, as we turned a corner, there was semi's (all without cargo) scatered all over the road. They were all on there sides or upside down as though they had been tossed on the road like dice. I looked back at the water and saw more semi's spread out and sinking as far as I could see. There was no crain helping them. When I woke up I was distressed, more so than I have ever been because of a dream. It was a worse feeling than having a nightmare. The rest of my sleep that night was broken and scattered, always waking to the memory of than dream.
What does it mean?